Custom QWidget with paintEvent implementation

main.cpp

#include <QtWidgets>

class Widget : public QWidget
{
public:
    Widget(QWidget *parent = 0) :
        QWidget(parent)
    {
        resize(300, 300);
    }

protected:
    void paintEvent(QPaintEvent *) Q_DECL_OVERRIDE
    {
        QPainter painter(this);

        QPen pen(Qt::red);
        pen.setWidth(10);
        painter.setPen(pen);

        const qreal halfPenWidth = pen.width() / 2;

        painter.drawRect(QRectF(halfPenWidth, halfPenWidth,
            width() - halfPenWidth * 2, height() - halfPenWidth * 2));
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    
    Widget w;
    w.show();

    return a.exec();
}